Field of Invention
This invention relates to a vehicle tire deformation device for determining the size of a deformed contact patch of a tire which contacts the ground. The invention further relates to a method of attaining the optimum tire pressure of a vehicle tire for the terrain and/or speed at which it is travelling by determining the size of a deformed contact patch of the tire when it contacts the ground. When the tire pressure and tire characteristics are known, the axle load can also be indirectly measured.
Description of Related Art
During normal use of an agricultural machine/vehicle, a part of the tire will always be in contact with the ground. This contact part of the tire will be flattened, that is the circumference of the wheel in contact with the ground will spread over the surface of the ground distorting the circular circumference of the tire. The size of the contact patch will depend on the tire pressure and the condition of the ground on which it is travelling, that is the hardness or softness of the ground. In order to improve efficiency and safety of an agricultural machine, or an agricultural vehicle such as an agricultural tractor, it is necessary to change the pressure of the tires depending on whether the tractor is operating, or on the road. When operating in the field, lower tire pressures are required to reduce ground pressure and compaction and to improve the grip of the tires with the earth. For road work, higher tire pressures are required to reduce rolling resistance (which affects the efficiency of the tractor) and to reduce heat generation (which affects the safety of the tractor). For a field of medium softness, a pressure between that of road and soft field is desired, so that the size of contact patch is between that of a soft field and a hard road. The pressures of the tires may typically be varied by 0.6 bar-2.5 bar when moving between field and road surfaces.
Generally, a tire inflation and deflation system comprises at least one rotatable air passage which is provided on, or within an axle to carry air to and from the tires. The rotatable passage may be connected to a further air duct for carrying air. The rotatable passage extends between rigid, stationary parts (which are connected to the vehicle frame, or form a part of the frame, for example an axle housing) and rotating parts (for example, the wheel hubs). Rotating parts such as wheel hubs are equipped with shaft seals to prevent oil from entering the air guiding area of the rotatable passage. Such systems are described in the Applicant's previous UK patent application Nos. GB1021928.5 and GB1021931.9.
It is difficult to measure axle load because axle load sensors are expensive and temperature sensitive.
It is known to use a polymer foil mounted on the inside of an outer tire to measure tire pressure. An example of such a foil is that used by Pd2m GmbH in its “S-Tire”. The pressure sensed by one or more foils within the tire can be used by a control system on the vehicle to set the correct tire pressure via a control unit. A change in pressure can be detected by a foil as it rotates through a contact patch. It is an aim of this invention to provide means for detecting the size of a deformed contact patch of a vehicle tire so that the axle load can be calculated. It is a further aim of the invention to provide a method for attaining an optimum tire pressure for the efficient operation of a vehicle depending on the terrain and/or speed upon, and/or at which a vehicle is travelling.